1. Field of the Invention
This invention relates generally to vehicular trucks, and more to a new and improved high lift tag axle for transit mixers, i.e. concrete mixer trucks and the like, which will not only permit an increase in the legal payload of the truck or mixer unit, but does not rely on a complicated hydraulic system to hold the tag axle in either the lowered or raised position. Rather, the tag axle of this invention utilizes a mechanical lock-down arrangement so that all ground forces are transmitted through the physical structure thereof and not the hydraulic system. In addition, the tag axle of this invention can be raised to an exceptionally high position so that it is not obstructive to complete access to the rear of the truck for the purpose of discharging concrete and the like.
2. The Prior Art
It is well known that most federal, state and local highways have load limits which limit the weight of vehicles traveling thereon. While there are total over-all weight limits per vehicle, many highways regulations also provide for load limits per vehicle axle. This is particularly true in the case of bridges where in addition to overall weight limits per vehicle, there are also weight limits to prevent localized weight concentrations. In this regard, the Federal Highway Administration has issued a "Bridge Gross Weight Formula", commonly referred to as the "bridge formula", which specifies load limits on the basis of bridge load carrying classifications, and provides formulas for determining load limits based not only on the basis of weight per axle, but also on the basis of distances between axles and the total over-all span between the front axle and the rearmost axle. The essence of the bridge formula is to prevent heavy loads from being concentrated on few closely spaced axles. Therefore, the total overall weight limit for many truck designs can be increased, to a limit, if the weight is spread over more axles spread further apart. While the federal bridge formula is applicable only to federal highway, many states have adopted the bridge formula into their own highway load limit regulations.
The use of auxiliary axles on trucks for providing additional load-bearing wheels is well known in the art. Particularly on tandem and tri-axle trucks, it is common practice to provide a hydraulic or spring actuated lift for one of the rear axles, so that it can or will be lifted from the road surface when the load is light and the additional load-bearing wheels are not needed.
In the case of transit mixers, i.e. concrete mixer trucks, there has been considerable interest and development in the art of "lift axles" or "tag axles", which are separate axle assemblies added to the rear of the truck. Such tag axles include a mechanism for lowering the unit so that the wheels will engage the road surface at some point behind the rearmost tandem axle, so that the legal load carrying capacity of the transit mixer can be significantly increased and yet satisfy the bridge formula. This increased load carrying capacity is effected not only by virtue of the fact that the load is spread over more axles, but also because the distance between the front and rearmost axles is significantly increased. Utilization of such a tag axle can permit a conventional tandem axle transit mixer, having a legal load limit of about seven cubic yard of concrete, to be legally increased to nine or even ten cubic yards.
Ideally, the tag axle is utilized to carry such heavy loads only when the transit mixer is traveling on public roadways and needed for the purpose satisfying the bridge formula or other load limits. In off-the-road use, however, such tag axles are not beneficial, and are therefore provided with means for lifting the tag axle from the roadway surface. In lifting the tag axle off the roadway, the load on the front axle is reduced to permit easier steering, the load is increased on the drive axles to increase traction, the truck's maneuverability is increased by reducing its overall wheel base, and the truck can be positioned closer to forms and other unloading sites to facilitate the discharge of concrete.
One disadvantage of some tag axles in current use, is that they cannot be raised sufficiently to get them out of the way to facilitate access to the discharge end of the truck. These tag axle units are based on folding linkage which hydraulically pulls the tag axle up under the discharge chute only inches from the road surface, so that the operator must work around the tag axle in discharging the concrete from the mixer. Other designs are known, however, which do raise the tag axle well above the discharge chute and above the head of the operator so that it is not an obstruction to the operator's discharge activities.
All the tag axles in current use utilize a hydraulic system for activating the tag axle from one position to the other, and in addition rely on the hydraulic system for maintaining the tag axle in contact with the road surface, if not also for maintaining the tag axle in its lifted, off the road position. In fact, many of the tag axle units available commercially, rely on the hydraulic system to provide a "spring" suspension or shock absorbing feature to the tag axle, so that the ground loads are transmitted to, and borne entirely by, the hydraulic system. These hydraulic systems must therefore be adapted to very heavy duty use, and typically include pressure regulating controls so that the "hold down" pressure applied to the tag axle is adjustable for controlling that pressure in proportion to the load weight on the truck. It is readily apparent that the demands on the hydraulic system are significant, as the system is constantly active and under excessive pressure while the tag axle is being utilized, if not also while the tag axle is in the raised, off the road position. Accordingly, such tag axles tend to be rather expensive, and subject to frequent hydraulic system difficulties, failure and repairs.